Stabilized Approach + Runway Excursion

FSF Student Chapter at Purdue University reexamines 2003–2010 data.

by Wayne Rosenkrans and Emily McGee | February 6, 2014

Research-based teamwork yielded positive results for the seven inaugural members of the Flight Safety Foundation Student Chapter at the Purdue University Department of Aviation Technology, says Scott Winter, the chapter’s first president and now assistant professor of aviation science at Florida Institute of Technology.1 Among findings of the chapter’s Foundation-assigned research project during the 2012–2013 academic year were risk factors that might help explain how runway excursions occur even when professional pilots meet the industry-accepted criteria for a stabilized approach.2

In October, Winter presented the main findings of this descriptive study during the FSF International Air Safety Summit (IASS), and he was interviewed by ASW with Lukas Rudari, the current chapter secretary, a Purdue graduate student and a summer 2013 intern at the International Civil Aviation Organization. The chapter was launched in September 2012.

“The student chapter is still going strong entering its second full year,” Winter said, citing the 11 currently active students’ work in progress on crew resource management problems as a causal factor in accidents worldwide. Rudari said, “It’s an enormous privilege to have access to all the resources of Flight Safety Foundation for, actually, a very affordable price. For students, money matters, so it’s a great opportunity for us to have access to AeroSafety World, to the databases and do research with that data.”

The project Winter presented at IASS began as a comparison of runway excursion analyses covering 2008–2010 events from an FSF dataset that matched those in a study by The Boeing Co. of 2003–2010 events involving only Boeing-related aircraft. Students first focused on the degree of similarity or difference in the comparison. As a follow-up effort, they successfully proposed to explore derivative questions with a different subset of FSF data comprising 520 runway-excursion accidents from 1995–2010.

“Essentially, we found out from the comparison that while the rank order may vary slightly, the top three risk factors tended to remain the same, contaminated runway, landing long or with excessive speed and unstabilized approaches,” Winter said. The team’s follow-up analysis included runway excursions in industry sectors other than major airlines. The students categorized events by type of operator (regional, air taxi, non-scheduled and corporate), aircraft type, aircraft class and key factors that apparently led to excursions after stabilized approaches, he said.

The team identified, for example, a “fairly consistent pattern” regardless of the type of operator, with — from highest to lowest frequency — contaminated runway, unstabilized approach, and landing long or landing outside the touchdown zone as the leading risk factors, followed in order by hard landings, tailwinds and thrust-reverser malfunctions, he said, adding, “If the approach is stabilized, you’re on speed, you’re on glide path, you’re maintaining descent rate, everything is going well. So why were these excursions occurring?” Winter asked. “At what point does something go wrong and cause the aircraft to have an excursion? We identified that there were 183 out of the 520 landing accidents — in the FSF database stretching from 1995 to 2010 — that resulted from excursions after stabilized approaches. One limitation is that within the database, about 50 accidents were ‘unknown’ in terms of the type of approach [stabilized/unstabilized].

“In at least in 130 of those 183 occurrences, no pilot factors were attributed to the runway excursion that occurred after the stabilized approach. When a pilot factor did occur, it typically revolved around a loss of directional control, poor crosswind compensation and poor speed control. So even though the approach was stabilized as the aircraft was coming in to land, perhaps something went wrong in the flare. The aircraft was floated, maybe it landed long, maybe there was just a little bit of excess speed. Maybe the crew attempted a smooth landing but, for whatever reason, they landed outside the touchdown zone, dealing with crosswinds and ultimately losing directional control of the aircraft.

“Even after the stabilized approach, a safe landing was not guaranteed. It was still necessary for the flight crew to maintain their awareness, maintain their profiles, and continue flying the aircraft all the way through the landing until the aircraft turns off at the gate. As some [IASS attendees remarked,] there is a lot of research being done on that right now. There are operational pressures, external pressures, aircraft malfunctions and still the opportunity for pilot error to occur during the final moments of the approach phase of flight.”

The team’s follow-up work also revealed a few mechanically related issues — for example, burst tire and thrust-reverser malfunctions — as well as landings outside the touchdown zone related to speed, contaminated runway and wind factors.

Winter said the authors tried to be careful not to overstate or speculate about the significance of these preliminary findings — or to suggest mitigations — about runway excursions after stabilized approach. Moreover, they recognized that, unlike the level of post-event time-factor accuracy researchers can achieve, it can be very difficult for a flight crew to recognize anomalies in real time.

In the IASS question-and-answer session, several attendees concurred that the Purdue University team had made a valid point about approaches/landings becoming “unstable at the last minute.” One said that airfield design/air traffic factors, such as pilots misjudging and missing high-speed turnoffs, add pressures to touch down and to keep a relatively high speed to the end of the runway.

In an FSF video interview, Winter and Rudari told Caren Waddell, consultant, FSF student chapters and projects, that participating in this student chapter and participating in the project itself had fulfilled the chapter’s goal of providing a better understanding of the dynamic safety issues in air transport.

Winter said, “Working with the Foundation has been really great. They’ve gone the extra mile every time, helping to start something that previously didn’t exist. In an FSF]student chapter you produce a deliverable, a research project — something that is really valuable to have when you are trying to differentiate yourself from a number of other students applying for the same position. It is a phenomenal opportunity. The people you are able to meet — to network with, to see the global issues that are going on — really help to just formulate in your mind what you’re doing at the university, what can pay off in the real world [in] contacts for future research, internships or jobs.”

Notes

Winter’s doctoral dissertation on pilot decision making in irreversible emergencies was accepted by Purdue University in May 2013.